The invention relates to a method for the manufacture of electronic semiconductor devices and, in particular, devices comprising an integrated circuit and power components on the same chip of semiconductor material.
It is known that in order to reduce the collector series resistance of the transistors of an integrated circuit, a buried layer may be provided below the collector region of these transistors. If this buried layer is provided on a heavily doped layer, a phenomenon layer as "out-diffusion" takes place, and the doping agent in the doped layer below the buried layer is outwardly diffused from the original layer. The resultant buried layer then becomes thicker than required and its electrical characteristics are modified. In addition, the collector region lying above the buried layer is modified in that undesired intermediate or "phantom" layers may be formed therein, particularly in the portion of this layer closest to the buried layer. The conductivity of these layers is opposite to the conductivity which should exist in the buried layer and the collector region lying above it.
The phenomenon of out-diffusion is particularly problematic in the case of a monolithic structure in which the components of the integrated circuit are isolated from one another and from the remainder of the substrate by "isolation" regions of a conductivity opposite to that of the substrate.
As a result of the adverse effects of out-diffusion, considerable attempts have been made to prevent or, at least, reduce them. In a known method, the phenomenon of out-diffusion is reduced by lowering the temperature at which the operations are carried out. In a further known method, doping agent concentrations, times and temperatures are controlled within a very strict range of variation. However, this makes the processes involved in the manufacture of the device very critical and industrial manufacture is not economically viable.
In the above-mentioned monolithic structures, the problem of out-diffusion is accompanied by the problem of providing a collector region of a power transistor and collector regions of other transistors of the integrated circuit so as to have different concentrations of a doping agent. This requirement may also be the case for the transistors belonging to the integrated circuit when different electrical properties are required for these transistors.
For example, a higher doping level is required in the collector regions of the transistors of the integrated circuit designed to operate with a low collector-emitter voltage at saturation.